As described in U.S. Pat. No. 6,411,531 to Nork et al., a pulse frequency modulation (PFM) charge pump regulator comprises a switch-capacitor network as a charge pump, in which switches are switched with a modulated switching frequency according to an output-dependent feedback signal, to connect a capacitor to a power supply or to an output terminal in order to charge and discharge the capacitor, so as to produce a regulated voltage on the output terminal. Such circuit will cause large input ripple and thereby large supply voltage disturbance, and induce undesired low frequency noise subsequently. It is not easy to process this low frequency noise, and very large and expansive filter is required to filter out the low frequency noise.
U.S. Pat. No. 6,411,531 to Nork et al. also described another charge pump regulator which has switches in a charge pump switched with a constant switching frequency, and a variable resistor inserted between the charge pump and a ground terminal to be controlled by an output-dependent feedback signal to vary the resistance thereof. This variable resistor limits the charge current to the capacitor in the charge pump and reduces the input current ripple. However, the current ripple in the discharge phase is still large. Because of the smaller input current in the charge phase, this circuit will cause smaller supply voltage disturbance. However, in the range of low frequency, for example the switching frequency, the supply voltage disturbance caused by the input current ripple is still large. The large input current ripple also causes high frequency noise during the blank period which has all the switches in the charge pump open.
U.S. Pat. No. 6,411,531 to Nork et al. proposed an improved charge pump regulator, which also operates with a constant switching frequency, but has a variable resistor inserted between the power supply and the input terminal of the charge pump. The resistance of the variable resistor is varied according to an output-dependent feedback signal in order to control the input current in each phase, so as to reduce the supply voltage disturbance that causes low frequency noise. However, the input series resistance is so increased, and thereby the power consumption is increased, resulting in decreased efficiency of the charge pump regulator.
Texas Instruments (TI) incorporated provides a product with serial no. TPS6500, which inserts a current source between the power supply and the input terminal of the charge pump for providing a stable input current, so as to reduce the input ripple. However, this current source is still an added element outside the charge pump.